People may find the bass on the ER4 to be lacking for different reasons. It may be because they are used to very bass heavy speakers and/or headphones with 10+ db of bass boost, because they are not getting a good seal with the IEMs, or because they have hearing loss in the lower frequencies. People, including myself, who do not find the bass on the ER4 to be lacking generally find that it does provide sufficient impact and extension. I can say that the sub bass is definitely there on the ER4 - quite audible and enjoyable, even for music like dance and hip-hop. I find that the ER4 has sufficient bass compared to other fairly neutral, accurate headphones like Sennheiser HD650 or Hifiman HE-500. I don't feel that the ER4 is missing much if any bass energy compared to those full sized headphones. In fact, ER4 has better extension and tighter bass definition than HD650 and is competent with planar magnetics like HE500 in bass quality.

When the ER4 is said to have no bass does this mean it is similar to having a hi-pass or lo-cut filter on everything below 100hz or so? Like similar to having a cheap radio like a construction worker held up to your ears?

Or, is the bass pretty good, but really fast and not so prominent because it is not so resonant? If so, Is this similar to how a decent mixing speaker like the auratone 5c's or Yamaha NS-10's bass responds?

When placed in the ear, is some of the resonating space in the ear canal creating additional bass response that may be unaccounted for if the specs are coming from tests conducted out of context or basically not measured in an inner ear environment?

It does sound more like a mixing speaker, with slight sub-bass roll off. All the bass details are there except for the lowest lows below 40Hz, where it starts to become difficult to detect.

I found a frequency response chart of the ER-4S, which is flatter than sh*t, but the "S" model requires an amp for best performance.

A ER-4P and maybe even a ER-4PT exists, which has a lower impedance, meant to be used without an additional amp, but would the frequency response be radically different? I can't find a frequency response chart, but I would be afraid that it would change and maybe even for the worse, where the "P" model would not have as good of response as the "S" model.(?) I would rather use a model without using extra gear like an amp.

The response would be different. If you add a 75 ohm (I think) resistor to the ER4-P, it becomes an ER4-S.

It does sound more like a mixing speaker, with slight sub-bass roll off. All the bass details are there except for the lowest lows below 40Hz, where it starts to become difficult to detect.

How so? I listen to a variety of bass music on these, like Mala, Goth-Trad, Hexan Cloak, etc. and these artist hit the lowest of low frequencies. The ER4 never misses a note, now matter how low it is. It has excellent extension, and most definitely hits below 40. Very detailed bass too. It just isn't really fat bass, which is why people say its bass deficient.

I mean to say the amplitude of bass at 30Hz isn't as loud as at 40Hz, which isn't as loud as at 60Hz. A sine sweep and looking at the graphs both show what I mean. It's not terribly bad, and I much prefer it to bass bloat apparent in most other iems.

For what you are looking at, it would have to be an impedance difference which basically has a variable resistance to different frequencies, so it is not as cut and dry as what you are saying. If you get into it, it gets more technical and mind-boggling, but worth it in the end.

It's not the resistor that's doing it, it's actually the driver that has variable resistance against different frequencies.